The field of the present invention relates generally to power switching circuits and devices, and more specifically to such circuits and devices, including the combination of bipolar and field effect transistors.
It is known in the prior art to apply metal oxide semiconductors (MOS devices) for use in low power digital circuits, and to apply power bipolar transistors for relatively high-power applications. In general, as the power handling capability of a bipolar transistor is increased, its gain must be increased but its switching speed will decrease. Also, bipolar transistors have a positive temperature coefficient of current, which can lead to "thermal runaway" as certain areas of the bipolar transistor's substrate heat up under severe operating conditions, causing damage or destruction of the transistor when it is operated in parallel with like devices. Recently, a vertical metal oxide semiconductor (hereinafter referred to as VMOS) field effect transistor has been invented. The VMOS device has a very high static input impedance, and consequently requires extremely low drive power, because it is a voltage operated device with high power gain. VMOS devices provide very fast switching times, permit direct paralleling of devices without complicated biasing networks for switching high current levels, and have a negative temperature coefficient for current (a positive temperature coefficient for resistance), thereby providing negative feedback internal to the device which substantially eliminates the destructive thermal runaway problem of bipolar transistors. In the present state of the art, VMOS devices are available for handling voltages up to 400 volts at about 8 amperes of current. Unfortunately, the present high power VMOS devices have a relatively high on-resistance of about 1 ohm between their source and drain electrodes. This resistance causes relatively high power dissipation at high power levels. For example, assuming that 100 ampere VMOS devices will become available in the near future, but with little improvement in the series on-resistance of the device, about 10,000 watts will be dissipated at this magnitude of current. In comparison, bipolar power transistors typically have a resistance of less than 20 milliohms between their collector and emitter electrodes when conducting 100 amperes in a saturated state, but have the disadvantage of a relatively low value of input resistance, relatively low switching speed in comparison to a VMOS transistor, and other problems as previously mentioned.